It's been awhile since I've dealt with optical imagery. I need to perform a land cover classification at mid-latitudes using several LANDSAT scenes. I've noticed that some online tools are still selling the imagery, but I was under the impression that the archive was now offered up at no cost.

7 Answers
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I am in Canada so if I need this imagery I can get it free at Geobase.
Elsewhere you should be able to download from USGS direct. You will need to register on both sites. Here are NASA links to download free Landsat data.

I'm also in Canada (also originally from northern Ontario) - Geobase is great for Canadian data, not so much for outside the country. That's for the USGS link - I just came across GloVis (glovis.usgs.gov), which looks like it allows users to search and download data after registering.
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Nick OFeb 15 '12 at 19:01

It only takes then to upload the .txt file in the above mentioned page (web-service) and get as a result down-loadable scenes (links) or submit a Bulk Download order which can be completed by using the Bulk Download Application.

Notes

the GLCF maintains a small subset of the entire Landsat archive, prior to 2006

[update] there is, however, (currently?) a restriction in downloading up to 100 500 Landsat scenes per Bulk Download Order

USGS is what I use.
Note that high quality processing can be achieved in GRASS, I encourage you to carry out atmospheric correction. There are a lot of "primers" out there about calculating NDVI etc which completely overlook atmospheric effects. You can achieve quantitative analysis this way.

I find the USGS NewEarthExplorer to be the easiest way to download Landsat scenes. As I discuss in a blog post, it has a far more 'modern' interface which makes it easier to select appropriate images for your needs.

The USGS have been recommending using EarthExplorer (EE) because it has the most development resources right now.

They offer the USGS Inventory Service as a machine-to-machine SOAP API for communicating with EE for querying their archive for scenes. This way, you can also order scenes for processing to Level 1 georegistered radiance products, and also get the weblink to download them.

A Python tool is being developed here LANDSAT-Download
where you can provide the Python script a list of WRS-2 target sites, and it will download all the available products .tar.gz for the year